The present invention relates generally to keyless entry systems. More particularly, the invention relates to a method for resynchronizing the transmitter/receiver pair when synchronization is lost due to momentary power failure or a low battery condition, or repeated manipulation of the transmitter buttons when the receiver is out of range, for example.
Rolling code authentication is a common form of vehicle entry security. In such a system, a transmitter is provided in the form of a key fob and a receiver is positioned in the vehicle where it is able to receive encoded transmission from the key fob transmitter. Rolling code authentication can be performed by employing a simple linear counter which advances with each key fob command. The receiver in the vehicle is configured to always expect an increasing value and therefore it disallows repeating counter values. Thus to be in sync the transmitter counter should never fall behind the count of the receiver, nor should the transmitter counter be permitted to get too far ahead of the receiver count. More complex authentication using linear shift feedback register (LFSR) technology is also used as a more secure technique for vehicle entry security.
For a number of reasons, a rolling code authentication system can occasionally fall out of synchronization when the counter values of the transmitter are less than that of the receiver or when the transmitter counter values are greater than those of the receiver by a predetermined number. Loss of synchronization can occur when the transmitter is repeatedly cycled (by pressing the key fob buttons) when the receiver is out of range. Loss of synchronization can also occur when battery power is lost.
One way to ensure against loss of synchronization due to battery power loss is to outfit the transmitter with a nonvolatile memory such as an EEPROM which can be used to store the rolling values so they will not be lost. Being nonvolatile, the EEPROM will not lose synchronization due to a power interruption (e.g. loose battery connection or battery failure). The EEPROM protects the integrity of the counters when the internal RAM is powered-off.
However, EEPROM devices are comparatively expensive and it would be desirable to eliminate them from the rolling code authentication circuitry. This presents a problem, since without nonvolatile memory, a system would have to rely on RAM (volatile memory) to store counter values. The need to rely on RAM increases the possibility of corrupted counter values, since even temporary loss of power through a loose battery connection or loss of battery charge would break synchronization.
Loss of synchronization due to repeated cycling of the transmitter when the receiver is out of range is a more difficult problem to address even with EEPROM devices, since eventually, the EEPROM device will become full and will thereby loose the ability to re-establish synchronization. For example, an EEPROM device with capacity to hold twenty numbers would loose synchronization on the twenty-first key press of the transmitter fob while out of range of the receiver. In effect, the twenty-first key press would cause the matching number to be lost as the twenty-first number is added.
It would therefore seem desirable to have a panic button function or resynchronization function which the user could invoke to force resynchronization in the event it is lost. Such a function is difficult to provide without sacrificing security, however. Care must be taken to ensure that the resynchronization sequence cannot be easily recorded and mimicked by a thief. If the resynchronization codes are easily mimicked, it would be a simple matter to gain entry to the vehicle by imitating the resynchronization sequence and then supplying the receiver with a known access code, in effect reprogramming the lock to match the key of the thief. Existing technology has not adequately addressed this problem.
Accordingly, the present invention provides a secure method of synchronizing transmitter and receiver in a keyless entry system of the type which uses encrypted access codes to prevent unauthorized access. The method comprises storing secret information data in the transmitter and storing the same secret information data in the receiver. The secret information includes a resychronization authorization code which is common to both transmitter and receiver. Preferably this resynchronization authorization code is preprogrammed into the transmitter and receiver units during manufacture or by the dealer or installer of the keyless entry system. Further in accordance with tile invention there is stored at least a first access code in the transmitter and at least a first access code in the receiver. These access codes serve to permit access if the transmitter and receiver first access codes match. The access codes further serve to prevent access if the transmitter and receiver first access codes do not match.
According to the inventive method, when a resynchronization sequence is initiated (e.g. by pushing a panic button or resynchronization button) a first random number access code is generated at the transmitter. The transmitter then transmits the resynchronization authorization code and the first random number access code to the receiver. In the transmitter, the first random number access code is substituted for the first access code. Meanwhile, in the receiver, the transmitted resynchronization authorization code is compared with the resynchronization authorization code stored in the receiver. If the transmitted resynchronization authorization code and the stored resynchronization authorization code match, a substitution is made whereby the first random number access code is substituted for the first access code in the receiver. In this way, the first access codes of the transmitter and receiver are reset to match one another, thereby synchronizing transmitter and receiver.
For a more complete understanding of the invention, its objects and advantages, reference may be made to the following specification and to the accompanying drawings.